1. Technical Field
The present invention relates to cubic silicon carbide film manufacturing methods, and cubic silicon carbide film-attached substrate manufacturing methods. Specifically, the invention relates to a cubic silicon carbide film manufacturing method that forms a cubic silicon carbide (SiC) film, an expected wide bandgap semiconductor, on a silicon substrate or on a monocrystalline silicon film formed on the substrate, and to a method for manufacturing a cubic silicon carbide film-attached substrate that includes a cubic silicon carbide film formed on a silicon substrate or on a monocrystalline silicon film formed on the substrate.
2. Related Art
Silicon carbide (SiC), a wide bandgap semiconductor having a bandgap of 2.2 eV (300 K) more than twice as large as that of silicon (Si), has generated interest as semiconductor material for power devices, or as material for high-voltage devices.
The crystal forming temperature of silicon carbide (SiC) is higher than that of silicon (Si), and obtaining silicon carbide (SiC) single crystal ingots by a pull method from a liquid phase is not as easy as in silicon. An alternative method, called a sublimation method, is thus used to form silicon carbide (SiC) single crystal ingots. However, it is difficult with the sublimation method to obtain large-diameter silicon carbide (SiC) single crystal ingots that have few crystal defects. This has limited the diameter of the currently available silicon carbide (SiC) substrates in the market to 3 to 4 inches, and has made the price of these products very expensive.
Cubic silicon carbide (3C-SiC), a variation of silicon carbide (SiC), has relatively low crystal forming temperature, and can be epitaxially grown (heteroepitaxy growth) on inexpensive silicon substrates. The heteroepitaxial technique has thus been studied as one way of increasing the diameter of silicon carbide (SiC) substrates.
The cubic silicon carbide has a lattice constant of 4.359 angstroms, about 20% smaller than the lattice constant (5.4307 angstroms) of monocrystalline silicon. This, combined with different coefficients of thermal expansion, makes it very difficult to obtain a high-quality epitaxial film that has few crystal defects.
Further, because the monocrystalline silicon and the cubic silicon carbide have different coefficients of thermal expansion, bending of the silicon substrate generates stress while the substrate is cooled to room temperature after the epitaxial growth of the cubic silicon carbide film. The stress translates into crystal defects in the cubic silicon carbide film. The adverse effect of such stress can be effectively avoided by lowering the epitaxial growth temperature.
Generally, epitaxial growth involves growth in a gas phase (CVD method). In the CVD method, the growth temperature can be lowered, for example, by (1) allowing growth under a high vacuum, or (2) by using a source gas that easily decomposes at low temperatures, or a source gas that has Si—C bonds. A drawback of lowering growth temperature is that it slows the growth rate.
As a countermeasure, a method has been proposed in which silicon source gas and carbon source gas are alternately flowed to enable formation of a cubic silicon carbide (3C-SiC) epitaxial film with few crystal defects at a practical growth rate (see JP-A-2001-335935).
While the method of the foregoing publication enables formation of an epitaxial film with few crystal defects with the alternately flowed silicon source gas and carbon source gas, the epitaxial growth temperature of the cubic silicon carbide (3C-SiC) remains at 1,200° C. to 1,300° C., a temperature range no different from the epitaxial growth temperatures of common cubic silicon carbides (3C-SiC). Thus, the different coefficients of thermal expansion cause stress while cooling the substrate, and the stress translates into crystal defects. It has thus been difficult to reduce the crystal defects of the cubic silicon carbide film.